NEODyS-2

Near Earth Objects - Dynamic Site
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FREQUENTLY ASKED QUESTIONS
  1. What is the criteria for an object to be called a Near-Earth Asteroid (NEA)?
  2. What is an astronomical unit (au)?
  3. How likely is an NEA impact?
  4. If an impact becomes likely is there anything that can be done about it?
  5. What is chi? Why is it equal zero in the observational data for the object I am interested in?
  6. Why is the orbital uncertainty information missing for the object I am interested in?
  7. How good are your absolute magnitude estimates?
  8. Why are there too few (or too many) digits in the orbital elements?
  9. I don't have an observatory code. What should I enter in the services forms?
  10. How often is your site updated?
  11. My question isn't here. Who can I ask?


What is the criteria for an object to be called a Near-Earth Asteroid (NEA)?
Perihelion less than 1.3 au. Roughly put, this means that all objects which can come within 0.3 au of the Earth comprise the list of NEA's.
What is an astronomical unit (au)?
The mean distance of the Earth from the Sun, about 93 million miles or 150 million kilometers. The lunar distance is 0.0026 au and the radius of the Earth is 0.000043 au.
How likely is an NEA impact?
Maybe more than you expect. (But not so high that you should spend your retirement savings early!) The current best estimate is that impacts large enough to cause a global catastrophe occur on average every few hundred thousand years. These impacts are the ones large enough to cause significant global climatic disruption, widespread crop failures and probable societal collapse. This translates into a probability of a little less than one chance in a few thousand of such an impact occurring during a typical human lifespan. Smaller objects, of course, are more numerous, and impact more frequently. Impacts capable of causing severe local or regional disasters occur once every few hundred years, depending on how you define "local" and "regional." Fortunately most of the Earth is uninhabited so the likelihood of a impact near a populated area is far less.
If an impact becomes likely is there anything that can be done about it?
This depends critically on the amount of warning and to a lesser extent on the composition and structure of the asteroid, but in principle deflection of impacting objects will be feasible.
What is chi? Why is it equal zero in the observational data for the object I am interested in?
This refers to the chi-square statistical quantity that is used to determine our cutoff for observational outlier rejection and recovery. We currently reject observations if chi**2 > 8 and we recover previously rejected observations if chi**2 < 7.5. The chi column in the observational data describes the quality of the observation, essentially the weighted circular error in arc-sec.
There are a very few unusual objects for which our automated routine for rejecting outliers in the observational data fails. In such cases the chi**2 test is not applied and outlier rejection is done on a manual basis. Of course, objects without uncertainty information are not subjected to automatic outlier rejection, so they fall into this category also. See the next question below.
Why is the orbital uncertainty information missing for the object I am interested in?
There are a very few unusual objects for which we are unable to compute a reliable orbit. For these we provide the orbit computed with at least one of the orbital elements held constant. In these cases the orbit should be considered highly unreliable, and for practical purposes the object is irretrievably lost.
How good are your absolute magnitude estimates?
Our computed absolute magnitudes are in excellent agreement with the computations of others in the field, but it is important to note that the source of our data is solely the photometry published by the Minor Planet Center with their astrometric observations. For the numbered asteroids (esp. those below 100), our computation can be quite different from the "official" IAU value because we have not included any photometric data not reported to the MPC with astrometry. The link to additional physical information will typically provide a better value in these cases. However, for unnumbered asteroids our result typically represents the best estimate that can be obtained with the available data.
Why are there too few (or too many) digits in the orbital elements?
Our HTML screen displays are designed for human readability. If you need machine precision (for machine readability), then you should download the ASCII files. However, the actual precision of the files is described by the "1-sigma variation" column in the HTML orbital element tables, which will generally be different from either of the element formats.
I don't have an observatory code. What should I enter in the services forms?
If you are doing only occasional observing, then you can use the default code of 500, or find an observatory near you and use their code. If you are intending to submit observations to the Minor Planet Center then you must contact them and obtain an observatory code. See their Guide to Minor Body Astrometry.
How often is your site updated?
The database is updated daily, and hence all orbital information is kept current.
My question isn't here. Who can I ask?
We would like to include your question here! Contact us and we will try to answer your question as promptly as possible.





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